1. Field of the Invention
The present invention relates to a device for controlling particle distribution in an evaporating droplet using radial electroosmotic flow, and more particularly, to a device for controlling particle distribution in a droplet, including: a first electrode developing an electric field in the vicinity of the droplet; and a second electrode that develops an electric field, is fixed to a substrate, is insulated from the first electrode, and is located at about the center of the droplet.
2. Description of the Related Art
The dynamics of an evaporating droplet on a solid substrate has been studied for heat transfer applications, and the effects thereof are commonly observed in everyday life, for example, in the ring-like stain that remains after a coffee droplet evaporates. Recently, this simple phenomenon has been determined to be an essential mechanism in biological and analytical chemistry applications, especially in the case of applications in which there is exposure to the atmosphere. The importance of the evaporation process for the precise detection in a protein chip and a genomic DNA microarray or the successful analysis of DNA and protein using MALDI-TOFMS has been demonstrated. However, unexpected ring-patterned stain formation after complete evaporation of a droplet has become a key obstacle to efficient analysis. Such localization of a solute may result in the need for time to search for a sample spot that yields an abundant signal. This search is labor-intensive and time-consuming, and hinders automation of the analysis. Thus, it is desirable to devise a method for homogenizing the solute distribution inside a droplet after complete evaporation.
The physiochemical mechanism of a drop stain phenomenon has yet to be completely understood, but contact line pinning due to a hydrophilic surface and the non-uniform evaporation from the edge of the droplet might contribute to the outward migration of a solute. As the droplet remains pinned on the substrate and the liquid is removed from the edge of the droplet first, the outward flow from the interior must replenish the mass loss. The explanation for evaporation from the edge of a droplet that is considered most reasonable is that the probability of an evaporating molecule escaping at the edge of the droplet is higher than the probability of an evaporating molecule escaping from the center of the droplet due to the geometrical curvature of the droplet.
One way to improve spatial stain homogeneity is to develop a system which uses a substrate other than a bare metal or chemical surface treatment. The use of poly-tetrafluoroethylene and polymer/Nafion substrates was reported to produce a relatively homogeneous solute distribution (Kim, Y.; Hurst, G. B.; Doktycz, M. J.; Buchanan, M. V. Anal. Chem. 2001, 73, 2617-2624). Another possible way to produce a relatively homogeneous solute distribution is to control outward radial flow inside the droplet. Stirring and spin-coating sample preparation methods have been reported to achieve the control (Westman, A.; Demirev, P.; Huth-Fehre, T.; Bielawski, J.; Sundqvist, B. U. R. Int. J. Mass. Spectrom. Ion Proc. 1994, 130, 107-115 and Perera, I. K.; Perkins, J.; Kantartzoglou, S. Rapid Commun. Mass Spectrom. 1995, 9, 180-187). However, the surface treatment and the sample preparation techniques have shortcomings which give rise to an uncontrollable deposition pattern during experimental processes. Therefore, there is a need for a coating method that maintains a uniform surface regardless of whether the surface is hydrophobic or hydrophilic, that removes non-uniformity commonly found during the spotting and coating, etc., and that is a proper coating method for a metal-coated surface.